The present invention pertains to a differential demodulator for VHF radio signals. More particularly, the present invention pertains to a demodulator for performing differential phase shift keying demodulation, for example differential 8 phase shift keying (D8PSK) demodulation.
VHF data radio is frequently used for communication with commercial airliners, to provide navigational and other critical information to the airliners from a ground based location. Movement of the airliner relative to the ground and thermal drift in the transmit and receive oscillators result in a Doppler error in the frequency of the received signal, as well as a Doppler derivative error. Because of the low losses required in demodulation, and because of the possible Doppler derivatives, pure differential demodulation does not meet system performance requirements. As a consequence, there is a need for an improved demodulation technique which can track the Doppler error and its derivatives, while also avoiding the problem of differential phase noise.
The present invention is a method of demodulating a received radio frequency signal, including a baseband signal with differential phase shift keying encoded data, to obtain the encoded data. The present invention is applicable to modulation of any reasonable phase order. Thus, although in the following the invention is described with reference to D8PSK demodulation, it is application generally to any D*PSK demodulation, where * represents any integer.
In accordance with the present invention, the radio frequency signal is received and detected, the symbol phase of the encoded data is established, and any Doppler error in the received signal is reduced. The envelope of the phase-tracked baseband signal is normalized, and the phase of the D8PSK encoded data within the signal is tracked to obtain raw output symbols. The raw output symbols are mapped in a nearest neighbor map into one of 8 possible values, and the differential phase angle between successive, i.e. time-adjacent, mapped symbols is computed and then converted into a binary format, providing the encoded data.
In accordance with a preferred embodiment of the present invention, the Doppler error in the received signal is reduced by estimating the Doppler error and multiplying the detected signal by the negative of the estimated Doppler frequency, i.e. a signal offset from the baseband frequency in the direction opposite to, and in an amount equal to, the estimated Doppler frequency. Preferably, the estimated Doppler frequency is determined by correlating a portion of the received signal, such as the start of message signal, in a plurality of filters matched to different frequencies, and using the frequency of the correlator filter with the largest output magnitude as the estimated Doppler frequency.
Likewise, in a preferred embodiment of the present invention, the envelope of the filtered and decimated baseband signal is normalized by converting in-phase and quadrature components of the envelope into angular representations, and converting the angular representations into normalized in-phase and quadrature components. In a preferred embodiment of the present invention, the phase of the D8PSK encoded data is tracked in a fourth order phase lock loop. In a particularly preferred embodiment, the fourth order phase lock loop is made up of two second order phase lock loops in tandem, with the bandwidth of the first loop being wide enough to prevent phase unlock. Preferably, the D8PSK encoded phase symbols are tracked by multiplying the angular representations by 8 so as to strip off the 8 phase modulation, converting the stripped angular representations into stripped in-phase and quadrature components, applying the stripped in-phase and quadrature components to the fourth order phase lock loop, and multiplying the phase lock loop output by the normalized in-phase and quadrature components.